This invention relates to a color encoding system utilizing a spatial color encoding filter and comb filter apparatus for producing color representative signals.
It is known that a striped spatial color encoding filter may be utilized to encode a plurality of colors onto a photosensitive surface such as black and white film in a film camera or the photosensitive electrode of an image pickup device in a television camera. One method of encoding colors in this fashion is to utilize an encoding filter assembly comprising a first grating of alternate cyan and transparent strips for encoding red light as amplitude modulation of a first carrier wave and second grating superimposed over the first grating comprising alternate transparent and yellow strips for encoding blue light as amplitude modulation of a second carrier wave. The gratings may have the same spatial frequency, i.e., the same line density of strips in a direction normal to the direction of the strips, with the gratings angularly disposed relative to the direction of the scanning lines such that when the imaged stripe pattern on the photosensitive electrode is scanned by an electron beam the red and blue color representative carrier waves have different frequencies. Thus, the carrier waves may be separately bandpass filtered for producing separate red and blue color representative signals. The overall transmissivity of the filter stripes may be selected such that a brightness signal is contained in the average transmission of the filter. This brightness signal may be bandpass limited to approximately 3 MHz. The stripe widths and angle of inclination of the stripes of each grating relative to the scanning direction may be selected such that the red carrier wave is at 3.5 MHz and the blue carrier wave is at 5 MHz, with a 500 KHz bandwidth allotted for the color sideband information of each color carrier wave. It can be seen that with a 3 MHz luminance signal the required frequency spectrum must extend to 5.5 MHz to include the brightness signal and two color carrier waves and their associated sidebands. The 5.5 MHz bandwidth requirement is such that it approaches the limit of the useful frequency response of available image pickup devices such as vidicon camera tubes. In addition, problems may be encountered in that the gamma characteristics of the pickup device for the relatively widely separated frequencies of the color carrier waves will be different, and the separate color carrier waves may not track each other with changes in scene illumination.
Another method of encoding colors utilizing a stripe filter to encode the light reaching the photosensitive electrode of an image pickup tube is to phase modulate a carrier wave periodically with different color information. In such a method several colors modulate different phases of a color carrier wave. A major problem encountered in a color encoding system utilizing phase modulation of a carrier wave is that nonlinearity of the optical system and the image pickup tube make it essential that a reference wave accompany the color information through the system in order to be available for use in demodulation of the phase modulated carrier wave because slight deviation of the phase between the signal and reference waves results in erroneous color signals being produced by the demodulation circuits. Various methods have been proposed for providing this reference wave such as, for example, a reference grating superimposed over the color encoding grating such that scanning of the imaged encoding filter pattern and grating pattern produces a reference wave component in the composite signal derived from the image pickup tube during scanning. This method has a disadvantage in that the reference grating blocks illumination and results in lower efficiency of light transmission by the optical apparatus. Phase modulated systems in general have a main disadvantage in that even a slight undesirable phase change of the reference wave with respect to the phase modulated color carrier wave results in erroneous color signals being generated.
Thus, while it is desirable to amplitude modulate a carrier wave with color information because of the insensitivity of the amplitude modulated color wave to phase changes brought about by system nonlinearity, it has been necessary to utilize separate spectral regions for this, resulting in an undesirably narrow banded luminance signal in order to include even two relatively narrow band color signals. Further, while a phase modulated color carrier can provide full color information in a smaller spectral range than an amplitude modulation system, the phase modulated carrier system is unsatisfactory in that it is susceptible to phase changes caused by system nonlinearity.
Accordingly, it is an object of this invention to provide a color encoding system in which full color information of a scene is included as amplitude modulation of a plurality of carrier wave components and their associated sidebands occupying a common range of frequencies.
It is another object of this invention to provide a color encoding system in which a plurality of colors are encoded and appear as amplitude modulation of a plurality of color carrier wave components, the plurality of different color representative components being separated from each other by means including comb filter apparatus.
A system is provided for producing signals representative of the color of a scene. A spatial color encoding filter comprising superimposed encoding stripe gratings is disposed such as to spatially separate different colored light directed to a photosensitive electrode of an image pickup device. The stripes of at least one grating are disposed angularly with respect to the stripes of any other grating and to the direction of the scanning lines of the image pickup device such that the composite signal including a plurality of amplitude modulated color representative carrier wave components derived from the image pickup tube as its photosensitive electrode is scanned includes an amplitude modulated color representative carrier wave component that is of different phase on successively scanned lines. Apparatus is coupled to the image pickup device for combining signals derived from successively scanned lines and for separating the color representative carrier wave components of the composite signal for producing separate signals each representative of a different color of the scene.
In another embodiment of the invention a color encoding black and white film camera is provided, the camera including a striped spatial color encoding filter assembly placed between the scene to be encoded and the black and white film. The filter assembly includes first and second superimposed encoding gratings having stripe widths and having the direction of their stripes angularly disposed from each other such that an image of the encoded film produces, when subsequently scanned, a composite signal including two amplitude modulated color representative carrier wave components having the same frequency.
A more detailed description of the invention is given in the following specification and accompanying drawings of which:
FIG. 1 is a block diagram showing one embodiment of a system for producing color representative signals;
FIGS. 2a, 2b and 2c are diagrams representative of the color encoding filter assembly utilized in the system shown in FIG. 1, and waveforms obtained at various points in the system shown in FIG. 1;
FIG. 3 is a block diagram showing another embodiment of a system for producing color representative signals;
FIGS. 4a, 4b and 4c are diagrams representative of the color encoding filter assembly utilized in the system shown in FIG. 3 and waveforms obtained at various points in the system shown in FIG. 3; and
FIG. 5 is a diagram of a camera for encoding light from a scene onto black and white film.